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Old 07-20-2010, 05:43 AM   #1
a10t2
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May 2010
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I've mentioned it in a few threads before, but since it affects so many home brewers I thought it was worth a new discussion. For the past year or so I've been tracking my FGs using both a hydrometer and a refractometer, and found that the correlation used in JavaScript calculators, included in software, etc. is wildly inaccurate (at least for me). I've come up with a much better correlation, and I'm curious to hear what other brewers think. In particular, if anyone else has both SG and Bx data for multiple batches, I'd love to see how well your findings correlate.

Blog post with more info: Toward a Better Refractometer Correlation


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Old 07-20-2010, 10:19 AM   #2
HairyDogBrewing
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How do you launch the open office version?
I don't see a document in the zip, just supporting files.

Here are some readings from recent batches.
OG OG brix FG FG brix Beersmith FG
1.052 13.0 1.010 6.4 1.009
1.045 11.2 1.009 5.0 1.005
1.045 11.0 1.010 5.2 1.006
1.049 12.0 1.016 6.6 1.012
1.046 11.5 1.008 5.6 1.008



 
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Old 07-20-2010, 06:47 PM   #3
a10t2
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Hmm, if it's downloading as a zip your PC may be doing some kind of processing. It's just a .ods file. (direct link) Maybe disable anti-virus software for a second? Or send me a PM with your email address and I'll email it to you.

Anyway, I plugged your data in and while it's an improvement, it isn't as good as I saw. The mean discrepancy for the old correlation was -2.9 points, and with the new one it's 0.7 points. The standard deviation is 3.8 points though, so it isn't a great fit.

One thing you might want to look into is that based on those five OGs, your "wort correction factor" looks to be 1.00, and you may have it set to the default 1.04. I don't know if you can change that in Beersmith.
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Old 07-21-2010, 01:56 AM   #4
HairyDogBrewing
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I turned off Norton Internet Security and I still can't see a .ODS file.

I don't have a lot of confidence in my readings.
They just don't make any sense sometimes.

 
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Old 07-21-2010, 07:11 PM   #5
Brewerforlife
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refractometer's are not very accurate for measuring finished
beer IMHO, even with the correction formula's.Great for measuring
wort and O.G during the brew. Just my .02$. Cheers!!!

 
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Old 07-22-2010, 12:13 AM   #6
ni*
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You based your fit on 12 datapoints. Without wanting to be discouraging (from my experience, this is definitely something that needs looking into), your fitting equation has nine terms. I don't think a close fit with the data actually indicates much when you have only slightly fewer terms in your equation than datapoints to fit. ie, you could likely fit completely random data fairly well with that many terms. I don't have sufficient background in statistics to calculate the statistical significance of a fit like this, but I suspect there isn't much of one. With that many terms the closeness of the fit becomes pretty meaningless.

The equation will accurately predict the datapoints you have, but I don't think it's statistically clear that it will predict future datapoints with much accuracy.

 
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Old 07-22-2010, 12:37 AM   #7
Reelale
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Yep, what he said. I agree totally.

 
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Old 07-22-2010, 01:28 AM   #8
a10t2
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Yup, overfitting is the main concern at this point. That's why I'm looking for more data. The only reason I'm sticking with the full cubic set so far is that it makes it easy to drop terms and play around with fits later on, rather than do a new regression every time. That the linear equation remains a pretty good fit is encouraging.

Also, with these ranges (1.036-1.106 OG, 1.007-1.022 FG) hopefully there won't be too much need for extrapolation.
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Old 08-06-2010, 09:12 PM   #9
ajdelange
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Here are some additional FG data. The first FG goes with the first nD etc. All nDs are corrected to 20 C (WITHOUT the use of the refractomer's ATC - don't use it for beer that has started to ferment!)

These (or rather the "points": 1000*(FG-1)) are fit quite nicely (r= 0.9499 - note that's r, not r-squared) by a linear function with an rms residual of 1.1 and a peak residual of -1.7 points which isn't half bad but 7 data points isn't a lot to draw global conclusions from. The fit is fg_points = -2046.4 + 1531.6*nD The beers include stout, ESB, bock,Vienna, weizen, Pils and alt.

FG's
1.009
1.009
1.02
1.011
1.013
1.01249
1.01525

nD's (Relative to Air)
1.34153
1.34237
1.34877
1.34429
1.34575
1.34363
1.34534

OG's
10.52
12.82
18.57
13.43
15.90
13.86
14.74

ABV's
4.20
5.45
7.44
6.20
6.68
5.63
5.68

[Edit] Added ABV and original gravity data. Note that OG's are calculated from ABV and true extract.

[Edit] If you want to convert the nD's to Brix you can use

Brix = ((4997.6025*nD - 21822.2749)*nD + 32237.6805)*nD - 16304.345

(Doubt all those decimal places are needed but thought I'd leave them in)

Reason: Brix polynomial corrected - noted that nDs are relative to air

 
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Old 08-09-2010, 02:03 AM   #10
a10t2
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Thanks for sharing, AJ. Based on that data set, it seems like a correlation could be worked out based only on the final refractive index. That doesn't intuitively make sense to me - in a three-phase solution, I would think that at least two combinations of sugar and alcohol levels could yield the same nD. Do you have the OG data for those beers?

I definitely can't get as good a fit to my data without the second independent variable. It isn't terrible though (r^2 = 0.78).


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